CLASS AMPHIBIA 507 



which the latter discharges over them as they are extruded. 

 The male then loses the clasping instinct and leaves the female. 

 The jelly which surrounds and protects the eggs soon swells 

 up through the absorption of water. Cleavage takes place as 

 indicated in Figure 424. Some of the cells, called macromercs 

 (Fig. 425, A, mg), are large becausfe of a bountiful supply of yolk; 

 others, the micromeres (mi), are smaller. A blastula (Fig. 425, 

 A) is formed by the appearance of a cavity, the blastoccel (bl. 

 coel), near the center of the egg. Gastrulation is modified in the 

 frog's egg because of the amount of yolk present. The dark 



FIG. 424. Segmentation of the frog's egg. (From Sedgwick's Zoology, 

 after Ecker.) 



side of the egg gradually grows over the lighter portion until 

 only a circular area of the latter, called the yolk plug (Fig. 425, 

 yk.pl), is visible. This gastrula contains two germ-layers, an 

 outer ectoderm (C, ect) and an inner entoderm (C, end). A third 

 layer, the mesoderm (C, mes), soon appears between the other two, 

 and splits into two, an inner splanchnic layer, which forms the 

 supporting tissue and musculature of the alimentary canal, and 

 an outer somatic or parietal layer, which forms the connective 

 tissue, muscle, and peritoneum of the body-wall. The cavity 

 between these two mesodermal layers is the coelom. 



Soon after gastrulation a groove called the primitive or 

 medullary groove (Fig. 425, B, md.gr) appears, on either side of 

 which is a medullary fold (md.f). The medullary folds grow 

 together at the top, forming a tube which later develops into the 

 brain and spinal cord of the embryo. The medullary groove lies 



